上松断層大木地区トレンチに出現した傾斜層の成因に関する古地磁気学的手法による検討  [in Japanese] Paleomagnetic Method Applied to a Suspected Deformed Sedimentary Layer Exposed at Oki Trench Site at the Agematsu Fault in Nagano Prefecture, Central Japan  [in Japanese]

Access this Article

Search this Article

Author(s)

Abstract

木曽山脈西縁断層帯の上松断層が走る大木地区で行われたトレンチ調査において, 2か所のトレンチ (S1トレンチとS2トレンチ) の壁面に現れた傾斜層の成因に関する古地磁気学的研究を行った. バルジ地形に沿って最大40°で傾斜した堆積層の成因について, 断層運動による変形か初生的な堆積構造であるかが問題となり, 古地磁気学的手法による検討を行った. トレンチ壁面において, 同一層準の傾斜部と水平部から定方位試料を採取して残留磁化測定と段階交流消磁実験を行った結果, 残留磁化は安定した一次成分と二次成分に区分できた. Q比は低く, 非履歴残留磁化/自然残留磁化の強度比は高いなどの磁化特性から, 一次成分は堆積残留磁化が起源として考えられた. 一次成分の磁化は, 傾斜層の傾動補正をすると集中度が悪くなることから, 傾斜層は初生的堆積構造であると考えられた. 二次成分は, S1トレンチとS2トレンチで磁化方向と集中度が有意に異なった. S1トレンチの残留磁化の二次成分は粘性残留磁化と見なすことができる. S2トレンチの残留磁化の二次成分の磁化方向は大きく東偏するが, 集中度は良く, 保磁力はS1トレンチのそれより小さいなどの磁化特性を持ち, 粘性残留磁化以外の磁化 (例えば雷による等温残留磁化) の影響で獲得された可能性が考えられる.

A paleomagnetic method was applied to decipher the origin of a dipping layer of the Agematsu fault at Oki trench sites (Si, S2). In S1 and S2 trenches, a sedimentary layer dipping up to 40 degrees was observed on the slope of bulge. It was discussed if the origin of dipping layer was due to tectonic deformation or to primary sedimentary structure. Oriented samples were collected from the same stratigraphic horizon. Alternating field (AF) demagnetization revealed that the remanent magnetization was divided into the stable primary component and the secondary component that could be cleaned by demagnetization at low AF levels. The primary component was estimated to be of DRM origin from low Qratio and high ARM/NRM ratio. The origin of dips of the sedimentary layer is considered to be primary sedimentary structure, because the scatter of the directions of remanent magnetization after the tilt correction is larger than that before the correction. There were large differences in the mean directions and the scatters of the secondary components between S1 and S2 trenches. In S2 trench, the secondary component showed less scatter than primary component and eastward deflection. The secondary component of S1 was regarded as VRM. There is a possibility that the origin of the secondary component in the S2 trench is due to lightning-induced remanent magnetization.

Journal

  • Journal of the Japan Society of Engineering Geology

    Journal of the Japan Society of Engineering Geology 48(1), 27-34, 2007-04-10

    Japan Society of Engineering Geology

References:  12

Cited by:  1

Codes

  • NII Article ID (NAID)
    110006277230
  • NII NACSIS-CAT ID (NCID)
    AN00026635
  • Text Lang
    JPN
  • Article Type
    Journal Article
  • ISSN
    02867737
  • NDL Article ID
    8788324
  • NDL Source Classification
    ZM49(科学技術--地球科学--地質)
  • NDL Call No.
    Z15-263
  • Data Source
    CJP  CJPref  NDL  NII-ELS  J-STAGE 
Page Top